Responses of the humid zone ecotype of the Nigerian West African Dwarf sheep to mixed infections with Haemonchus contortus and Trichostrongylus colubriformis.

The responses of the Nigerian West African Dwarf sheep to experimental infections with two of its most important gastrointestinal (GI) nematodes, namely, Haemonchus contortus and Trichostrongylus colubriformis were studied by means of two measures of parasitological response, namely, faecal egg count (FEC) and worm burden (Wb), and three measures of host pathology, namely, packed cell volume (PCV), body weight (Bwt) and body condition score (BCS). Following exposure to weekly escalating infections (60% H. contortus and 40% T. colubriformis) starting with single doses of 500 infective larvae (L(3)) at week 1, 1,000 L(3) at week 2, 2,000 L(3) at week 3 and 4,000 L(3) at week 4, two distinctive worm burden response phenotypes were readily recognisable, namely, low Wb (LWb) and high Wb (HWb) phenotypes. The percentage of inoculums (adults and immature stages) recovered at necropsy were 3.75% and 33.08% respectively for H. contortus and T. colubriformis. The results showed that the FECs of the lambs belonging to the LWb phenotype were significantly lower than the FECs of their HWb counterpart. Among the measures of host pathology tested, the LWb phenotype sheep had similar Bwt, PCV and BCS to the uninfected control sheep, whereas the HWb phenotype sheep had significantly lower values than their LWb counterparts and the control sheep towards the end of the experiment. There was a highly significant positive correlation between worm burden transformed as log(10) (Wb + 10) and the average of D56 and D59 FECs. A highly significant negative correlation also was obtained between log(10) (Wb + 10) and the PCV, Bwt and BCS. The low percentage of H. contortus inoculums recovered at necropsy suggested that the sheep may be resistant to their most important GI nematode, H. contortus but less so to T. colubriformis.